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  1.  
    Hi there,

    I'm finishing off my final year dissertation titled "Energy Storage for 100% renewable electricy in Scotland". I have compared all technologies and Pumped Hydro seems to be the most suitable. If all Hydro stations in Scotland where converted to pumped hydro there would be a capacity of 530GW. Scotland consumes 140TW per year there for we would have around 1 1/2 days worth of storage if there was no energy generation. What I'm trying to work out is how much extra storage would we need to become self sufficient for renewable electricity. I'm guessing I would need to find out what would be the worst case scenario would be i.e how much energy would need to stored to cover days of low renewable generation. I can't find any research on this Issue. If anyone could help me work this out or point me in the right direction that would be great.

    Thanks
    Connor
    • CommentAuthorEd Davies
    • CommentTimeJan 26th 2016
     
    I think you mean 530 GWh storage and 140 TWh consumption per year but that's only a guess.
  2.  
    <blockquote><cite>Posted By: Ed Davies</cite>I think you mean 530 GWh storage and 140 TWh consumption per year but that's only a guess.</blockquote>

    Yeah your right! oops!
    • CommentAuthorEd Davies
    • CommentTimeJan 26th 2016
     
    The OpenEnergyMonitor chaps have been doing an exercise of this sort - modelling the whole UK on an hour-by-hour basis to see what storage would be required. I haven't read this stuff yet (busy catching up on the blog of one of their mates first - reading just a little quicker than he writes) but this might get you started:

    http://openenergymonitor.blogspot.co.uk/2015/08/understanding-zero-carbon-energy.html

    The idea of just switching to renewables and just putting a lot more electrical storage to compensate for indeterminacy seems a bit unrealistic to me. I _think_ (but am not absolutely sure) that a very-nearly completely renewable energy NW European country is possible but it'd take more than simple substitution of electricity sources: demand reduction, demand management, more electrical storage, more storage of energy in other forms (e.g., heat).

    It gets really hard to see if or how well they'd work together. So if you come to the conclusion that the amount of extra storage required for simple substitution is unrealistic then please at least add the caveat that it's possible that other changes could reduce the requirement substantially.
  3.  
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13999&page=1#Item_1

    Read the paper that djh has linked to in the above thread and it will at least back up your conclusion that stored hydro is the best storage option.
    • CommentAuthorbillt
    • CommentTimeJan 27th 2016
     
    It's just hopelessly unrealistic. There just aren't enough suitable sites in the UK to store the enormous amounts of energy needed.

    Where are you going to store all the water for these hydro conversions? Build a wall around Scotland maybe.
    •  
      CommentAuthordjh
    • CommentTimeJan 27th 2016
     
    I saw something the other day about another attempt to get the idea of a single electricty grid in the USA accepted, as a precursor to building a wide-area renewables grid with sufficient scale to be practicable. Specifically, you need a long north-south connection between wind systems if you want to be able to rely on them for continuous power, because the weather systems travel west-east. A planet-sized grid could actually get useful quantities of solar power in the winter. It all makes the idea of orbiting solar power stations with microwave downlinks sound feasible.
    • CommentAuthorringi
    • CommentTimeJan 27th 2016
     
    Large electrical grids are not the way to go, they are impossible to control and can be very unstable.

    However you can use DC links to connect independent grids together in a controlled way. A DC link allows you to control how much power flows, until a AC link where the power just takes the easiest path.

    DC link also allow the use of underground and undersea cables, and are coming down in price, as the power electronics are coming cheaper. (The capacitance on underground AC cables are go long for long runs.)

    Yet the UK still don't have enough links from Scotland to the source so that all the wind power can be used!
    •  
      CommentAuthordjh
    • CommentTimeJan 27th 2016
     
    Yes, the US proposal is for a HVDC grid. As you say, there are limits to the sizes of synchronised AC grids.
    • CommentAuthorringi
    • CommentTimeJan 27th 2016
     
    Storage can be done if you don’t mind the loses from the storage system.

    It is not that hard to convert electrical power into methane, the methane can then be stored and used later to run gas turbines. However it is easier to put it into the gas grid, hence reducing how much gas is taken from the ground. The gas grid can store a lot of gas due to “line packing”.

    Selling the power to the England, then buying it back could be considered a form of storage.

    Methanol can also be made from electrical power and stored for a long time.

    The issue is not the storage systems; it is that renewable electricy costs too much! If the cost of renewable electricy was 25% of gas, then storage would be cost effective.

    There are lots of ways to reduce the demand in the short term, by remote control of freezers etc, but that only helps spread out the peak demand.

    We can have many days with no wind, or months without enough rain for hydro, so a storage system needs a VERY high capacity and storage cost should be considered a cost of using wind power etc. But tidal power is a lot more predictable, so maybe we should be investing more in tidal power.
    • CommentAuthorEd Davies
    • CommentTimeJan 27th 2016
     
    Posted By: billtIt's just hopelessly unrealistic. There just aren't enough suitable sites in the UK to store the enormous amounts of energy needed.
    The OP has already worked out the amount of hydro storage available in Scotland. He now wants to work out how much extra storage (non-hydro, presumably) would be needed. How do you know what sites would be suitable for that? What are the site-specific requirements of lithium ion batteries, do you think? Roughly horizontal, solid and dry would seem to be most of the requirement to me.

    Posted By: ringiYet the UK still don't have enough links from Scotland to the source so that all the wind power can be used!
    A good reason to study storage in Scotland.
    • CommentAuthorbillt
    • CommentTimeJan 27th 2016
     
    AFAIKS he hasn't done anything of the sort. He has suggested that if you could convert all the existing hydro plants to use stored water you would get 530GWhr of storage, but you have to be able to store the water somewhere, which he doesn't seem to have addressed.

    According to Wikipedia there is about 2.8Gw of installed pumped storage generating capacity in the UK, most of which (2GW) is in Wales. The biggest scheme, Dinorwig has only 9GWh of capacity. The total UK pumped storage capacity is about 30GWh, somewhat short of 530GWh.
    • CommentAuthorCWatters
    • CommentTimeJan 27th 2016
     
    This looks relevant but I haven't read it all..

    http://www.gov.scot/resource/doc/328702/0106252.pdf

    They seem to be saying that the driving force for storage is not just the intermittent nature of generation but to reduce the frequency and costs of constraining excess generation...

    Selected Quotes..

    "Scenario 2: Reflects more recent views on growth rates for renewable energy, with greater growth in the offshore section."

    "Under Scenario 2, which has higher volumes of intermittent generation, demand would be exceeded by supply for 28% of the year in 2030. Even with all thermal generation constrained generation would exceed demand for 15% of the year."

    "Under Scenario 2 it was found that there is a significant storage requirement needed in 2030. Considering the case of Scenario 2 Figure 6.5 showed that in order to reduce the amount of time constraints are required upon generation to 10% then approximately 7,000 MW of storage is required. To put this figure into context the current size of pumped storage schemes in Scotland and those planned is around 400MW. Installing 7,000MW of storage would be a massive engineering undertaking and would equate to over seventeen 400MW plants."

    "In summary bulk storage technologies cannot provide the full level of storage required under Scenario 2 for 2030."

    "The modelling work found that with high growth of intermittent renewables as described under Scenario 2 energy storage will be required from 2020 onwards. By 2030 under this Scenario there is a maximum power flow of 13 GW with an excess of approximately 5 GW even following upgrades to the interconnectors between Scotland and England."

    End quotes

    I think all this is saying that they definitely will have to constrain output much more than 10%.
  4.  
    Compressed air bags under the ocean. That's the future!
    •  
      CommentAuthordjh
    • CommentTimeJan 27th 2016
     
    I read that there are plans to put 23 airbags into each car to meet new rules. Perhaps they could all be plugged in to the grid and used to store energy? Even ICE-powered cars could contribute to our sustainable future.
    • CommentAuthorEd Davies
    • CommentTimeJan 27th 2016
     
    Posted By: billtAFAIKS he hasn't done anything of the sort. He has suggested that if you could convert all the existing hydro plants to use stored water you would get 530GWhr of storage, but you have to be able to store the water somewhere, which he doesn't seem to have addressed.
    If he hasn't addressed where the water would be stored why would he have a limit on the capacity at all? My guess would be that he's just taken the difference between the “empty” and ”full” states of the existing reservoirs but that's rather irrelevant to his question.

    My understanding, though, is that pumped storage becomes progressively less efficient the lower the head height is. Can't quite put my finger on why that is but I think it's why a lot of the existing hydro is not currently used for pumped storage. With a market with wider swings in electricity prices use of lower-head pumped storage might start to make sense.
    • CommentAuthorringi
    • CommentTimeJan 27th 2016
     
    pumped storage, need storage at the bottom as well as the top. Most hydro don't have the option of creating storage at the bottom.
    • CommentAuthorEd Davies
    • CommentTimeJan 27th 2016
     
    Good point Ringi, though a quick look at a few of the bigger ones shows them discharging not that far above Loch Ness which contains some water.
    • CommentAuthorringi
    • CommentTimeJan 27th 2016
     
    a) Is the water in Lock Ness OK to put in the top lake?
    b) The distance the discharge is above Lock Ness will be lost power on each storage cycle.

    There was a "design" a long time ago to use wind driven pumps to refill the top storage, these wind pumps being wind turbines connected to pump without any grid connections. Hence reducing costs and allowing the hydro to run for more hours each day before it runs out of water.
  5.  
    Thanks for the input guys! Reading through the papers you have posted. Regarding the pumped hydro figures. I found them from a study done by
    strathclye.http://www.esru.strath.ac.uk/EandE/Web_sites/03-04/wind/content/storage%20available.html
  6.  
    Posted By: billtIt's just hopelessly unrealistic. There just aren't enough suitable sites in the UK to store the enormous amounts of energy needed.

    Where are you going to store all the water for these hydro conversions? Build a wall around Scotland maybe.


    Problem was solved over a century ago with high pressure hydraulic accumulators just a shame the systems where abandoned in the seventies and eighties. Maybe they will return like trams which where also written off as backwards technology with lines ripped up.
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2016
     
    Posted By: renewablejohn: “high pressure hydraulic accumulators”

    Assuming a compressed-gas (rather than a spring or weight) I can't see how they can be at all efficient as you're bound to lose the heat from compression of the gas - it's diffused throughout the storage volume. Also, you have to build a vessel capable of holding the pressure of the gas for the whole of its volume plus the volume of the hydraulic fluid used to compress it.

    If you're going to use compression for energy storage then just compressing a gas directly seems like a much better approach as it's then possible to recover the heat and store it separately (e.g., in phase-change materials) for use directly as heat or to help with the expansion when you want to recover the energy. This is the approach LightSail are using:

    http://www.lightsail.com/
    •  
      CommentAuthordjh
    • CommentTimeJan 28th 2016
     
    Posted By: renewablejohnProblem was solved over a century ago with high pressure hydraulic accumulators

    Are they cost effective at the scale required? 140 TWh times however many weeks you think the wind might fail.
  7.  
    Posted By: Ed DaviesPosted By: renewablejohn: “high pressure hydraulic accumulators”

    Assuming a compressed-gas (rather than a spring or weight) I can't see how they can be at all efficient as you're bound to lose the heat from compression of the gas - it's diffused throughout the storage volume. Also, you have to build a vessel capable of holding the pressure of the gas for the whole of its volume plus the volume of the hydraulic fluid used to compress it.

    If you're going to use compression for energy storage then just compressing a gas directly seems like a much better approach as it's then possible to recover the heat and store it separately (e.g., in phase-change materials) for use directly as heat or to help with the expansion when you want to recover the energy. This is the approach LightSail are using:

    http://www.lightsail.com/" rel="nofollow" >http://www.lightsail.com/


    System used water just the same as pumped hydro schemes but at far higher pressures than normal hydro at between 50 and 70 bar.
  8.  
    Posted By: djh
    Posted By: renewablejohnProblem was solved over a century ago with high pressure hydraulic accumulators

    Are they cost effective at the scale required? 140 TWh times however many weeks you think the wind might fail.


    Exceedingly cost effective as the accumulator can be recharged from any surplus renewable energy source wind, solar, hydro, tidal etc. Your not actually transferring volume of water your actually transferring energy in the form of pressure and recovering that energy by pressure reduction technology. Already being used extensively in the gas industry but more efficient if used with liquids.
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2016
     
    Posted By: renewablejohnSystem used water just the same as pumped hydro schemes but at far higher pressures than normal hydro at between 50 and 70 bar.
    Right, but pumped hydro doesn't lose energy by compressing, and therefore heating, the atmosphere.

    (It does, of course, lose a tiny amount of energy displacing air between the volumes above the two reservoirs but that's just a tiny bit of its general inefficiency.)

    140 TWh/year comes to about 20 PJ/fortnight (peta (10¹⁵) joules).

    Decompressing 1 m³ of water from 70 bar (7 MPa (megapascals)) yields 7 MJ. So to store a fortnight of Scotland's energy use you'd need 20e15/7e6 = 2.8 Gm³ or a cube about 1.4 km on a side. That's ignoring the fact that you'd lose pressure as the water flows out. Still, huge but not completely impossible particularly if you decide you don't really need a fortnight's worth.
  9.  
    <blockquote><cite>Posted By: Ed Davies</cite><blockquote><cite>Posted By: renewablejohn</cite>System used water just the same as pumped hydro schemes but at far higher pressures than normal hydro at between 50 and 70 bar.</blockquote>Right, but pumped hydro doesn't lose energy by compressing, and therefore heating, the atmosphere.

    (It does, of course, lose a tiny amount of energy displacing air between the volumes above the two reservoirs but that's just a tiny bit of its general inefficiency.)

    140 TWh/year comes to about 20 PJ/fortnight (peta (10¹⁵) joules).

    Decompressing 1 m³ of water from 70 bar (7 MPa (megapascals)) yields 7 MJ. So to store a fortnight of Scotland's energy use you'd need 20e15/7e6 = 2.8 Gm³ or a cube about 1.4 km on a side. That's ignoring the fact that you'd lose pressure as the water flows out. Still, huge but not completely impossible particularly if you decide you don't really need a fortnight's worth.</blockquote>

    1.4 km is nothing compared to the miles of hydraulic systems that used to be in existence.

    https://en.wikipedia.org/wiki/Hydraulic_power_network

    I think Antwerp and Geneva are examples of what could be achieved with Geneva being particularly forward thinking using hydro power to store energy for further use with the safety valve becoming the tourist attraction which Geneva is famous for.

    What I find fascinating about the Geneva plant is the head height is only 2-4 mtrs well below the range many consider economic.
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2016
     
    A cube 1.4 km on a side is considerably bigger than quite a few hundreds of miles of hydraulic pipe.

    From what I understand, though, those hydraulic systems had very little actual storage of energy - minutes of operation or so. Fascinating about the Geneva fountain but an indication of the lack of storage as the evening shutdown caused such a spurt.
  10.  
    Posted By: Ed DaviesA cube 1.4 km on a side is considerably bigger than quite a few hundreds of miles of hydraulic pipe.

    From what I understand, though, those hydraulic systems had very little actual storage of energy - minutes of operation or so. Fascinating about the Geneva fountain but an indication of the lack of storage as the evening shutdown caused such a spurt.


    Assuming a pipeline cross section area of 1mtr length would be 68 times round the world. Does not seem very far to me. As for storage capacity its generally around 60% of energy input can be extracted and time is not a factor as you can dictate the speed of energy decline by speed of energy extraction.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 1st 2016 edited
     
    Had a quick look on Gridwatch and it seems that pumped storage accounts for about 1% of our generation capacity.
    This was just a quick calculation working with our demand and what pumped storage delivered every week.
    So to 'run the grid' from pumped storage only would imply it needs to be 100 times larger.

    During our consistently lowest demand week (week 33, 30,757 GW mean demand), pumped storage delivered 0.998%.
    During our consistently highest demand week (week 4, 41,409 GW mean demand), pumped storage delivered 0.879%.

    During the lowest demand week, wind supplied 1,740 GW, hydro (non pumped) 304 GW, 'Other' 466 GW.
    During the highest demand week, wind supplied 2,192 GW, hydro (non pumped0 549 GW, 'Other' 831 GW.

    Now that is for the UK in total, not got the figures for just Scotland.
   
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